The present invention relates to actuator elements such as may be used for active vibration reduction, structural control, dynamic testing, precision positioning, motion control, stirring, shaking, and passive or active damping. More particularly, the present invention relates to a packaged actuator assembly that is electronically controllable and may be used separately or adapted to actively suppress vibration, actuate structures, or damp mechanical states of a device to which it is attached. As described in a subsequent section below, the assembly may be bonded or attached to a structure or system, thereby integrating it with the system to be actuated, controlled or damped.
Smart materials, such as piezoelectric, electrostrictive or magnetostrictive materials, may be used for high band width tasks such as actuation or damping of structural or acoustic noise, and also for precision positioning applications. Such applications frequently require that the smart material be bonded or attached to the structure that it is to control. However, general purpose actuators of these materials are not generally available, and typically a person wishing to implement such a control task must take raw, possibly non-electroded, smart material stock, together with any necessary electrodes, adhesives and insulating structures and proceed to fasten it onto, or incorporate it into, the article of interest.
For such applications, it becomes necessary to connect and attach these materials in such a way that the mechanical and electrical connections to the smart material are robust and capable of creating strain within the smart member or displacing or forcing the system, and to couple this strain, motion or force to the object which is to be controlled. Often, it is required that the smart material be used in a non-benign environment, greatly increasing the chances of its mechanical or electrical failure.
By way of example, one such application, that of vibration suppression and actuation for a structure, requires attachment of a piezoelectric element (or multiple elements) to the structure. These elements are then actuated, the piezoelectric effect transforming electrical energy applied to the elements into mechanical energy that is distributed throughout the elements. By selectively creating mechanical impulses or changing strain within the piezoelectric material, specific shape control of the underlying structure is achievable. Rapid actuation can be used to suppress a natural vibration or to apply a controlled vibration or displacement. Examples of this application of piezoelectric and other intelligent materials have become increasingly common in recent years.
In a typical vibration suppression and actuation application, a piezoelectric element is bonded to a structure in a complex sequence of steps. The surface of the structure is first machined so that one or more channels are created to carry electrical leads needed to connect to the piezoelectric element. Alternatively, instead of machining channels, two different epoxies may be used to make both the mechanical and the electrical contacts. In this alternative approach, a conductive epoxy is spotted, i.e., applied locally to form conductors, and a structural epoxy is applied to the rest of the structure, bonding the piezoelectric element to the structure. Everything is then covered with a protective coating.
This assembly procedure is labor intensive, and often involves much rework due to problems in working with the epoxy. Mechanical uniformity between different piezoelectric elements is difficult to obtain due to the variability of the process, especially with regard to alignment and bonding of the piezoelectric elements. Electrical and mechanical connections formed in this way are often unreliable. It is common for the conductive epoxy to flow in an undesirable way, causing a short across the ends of the piezoelectric element. Furthermore, piezoelectric elements are very fragile and when unsupported may be broken during bonding or handling.
Another drawback of the conventional fabrication process is that after the piezoelectric element is bonded to the structure, if fracture occurs, that part of the piezoelectric element which is not in contact with the conductor is disabled. Full actuation of the element is thereby degraded. Shielding also can be a problem since other circuit components as well as personnel must generally be shielded from the electrodes of these devices, which may carry a high voltage.
One approach to incorporating piezoelectric elements, such as a thin piezoelectric plate, a cylinder or a stack of discs or annuli, into a controllable structure has been described in U.S. Pat. No. 4,849,668 of Javier de Luis and Edward F. Crawley. This technique involves meticulous hand-assembly of various elements into an integral structure in which the piezoceramic elements are insulated and contained within the structure of a laminated composite body which serves as a strong support. The support reduces problems of electrode cracking, and, at least as set forth in that patent, may be implemented in a way calculated to optimize structural strength with mechanical actuation efficiency. Furthermore, for cylinders or stacked annuli the natural internal passage of these off-the-shelf piezo forms simplifies, to some extent, the otherwise difficult task of installing wiring. Nonetheless, design is not simple, and fabrication remains time-consuming and subject to numerous failure modes during assembly and operation.
The field of dynamic testing requires versatile actuators to shake or perturb structures so that their response can be measured or controlled. Here, however, the accepted methodology for shaking test devices involves using an electromechanical motor to create a linear disturbance. The motor is generally applied via a stinger design, in order to decouple the motor from the desired signal. Such external motors still have the drawback that dynamic coupling is often encountered when using the motor to excite the structure. Furthermore, with this type of actuator, inertia is added to the structure, resulting in undesirable dynamics. The structure can become grounded when the exciter is not an integral part of the structure. These factors can greatly complicate device behavior, as well as the modeling or mathematical analysis of the states of interest. The use of piezoelectric actuators could overcome many of these drawbacks, but, as noted above, would introduce its own problems of complex construction, variation in actuation characteristics, and durability. Similar problems arise when a piezoelectric or electrostrictive element is used for sensing.
Thus, improvements are desirable in the manner in which an element is bonded to the structure to be controlled or actuated, such that the element may have high band width actuation capabilities and be easily set up, yet be mechanically and electrically robust, and not significantly alter the mechanical properties of the structure as a whole. It is also desirable to achieve high strain transfer from the piezoelectric element to the structure of interest.
An actuator assembly according to the present invention includes one or more strain elements, such as a piezoelectric or electrostrictive plate or shell, a housing forming a protective body about the element, and electrical contacts mounted in the housing and connecting to the strain element, these parts together forming a flexible card. At least one side of the assembly includes a thin sheet which is attached to a major face of the strain element, and by bonding the outside of the sheet to an object a stiff shear-free coupling is obtained between the object and the strain element in the housing.
In a preferred embodiment, the strain elements are piezoceramic plates, which are quite thin, preferably between slightly under an eighth of a millimeter to several millimeters thick, and which have a relatively large surface area, with one or both of their width and length dimensions being tens or hundreds of times greater than the thickness dimension. A metallized film makes electrode contact, while a bonding agent and insulating material hermetically seal the device against delamination, cracking and environmental exposure. The bonding agent used may be an epoxy, such as B-stage or C-stage epoxy, a thermoplastic, or any other material useful in bonding together the piezoceramic plate, metallized film and insulating material. The specific bonding agent used will depend on the intended application of the device. In a preferred embodiment, the metallized film and insulating material are both provided in a flexible circuit of tough polymer material, which thus provides robust mechanical and electrical coupling to the enclosed elements. Alternatively, the metallized film may be located directly on the piezoceramic plate, and the insulating material may have electrical contacts.
By way of illustration, an example below describes a construction utilizing rectangular PZT plates a quarter millimeter thick, with length and width dimensions each of one to three centimeters, each element thus having an active strain-generating face one to ten square centimeters in area. The PZT plates are mounted on or between sheets of a stiff strong polymer, e.g., one half, one or two mil polymide, which is copper clad on one or both sides and has a suitable conductive electrode pattern formed in the copper layer for contacting the PZT plates. Various spacers surround the plates, and the entire structure is bonded together with a structural polymer into a waterproof, insulated closed package, having a thickness about the same as the plate thickness, e.g., 0.30 to 0.50 millimeters. So enclosed, the package may bend, extend and flex, and undergo sharp impacts, without fracturing the fragile PZT elements which are contained within. Further, because the conductor pattern is firmly attached to the polymide sheet, even cracking of the PZT element does not sever the electrodes, or prevent actuation over the full area of the element, or otherwise significantly degrade its performance.
The thin package forms a complete modular unit, in the form of a small xe2x80x9ccardxe2x80x9d, complete with electrodes. The package may then conveniently be attached by bonding one face to a structure so that it couples strain between the enclosed strain element and the structure. This may be done for example, by simply attaching the package with an adhesive to establish thin, high shear strength, coupling with the PZT plates, while adding minimal mass to the system as a whole. The plates may be actuators, which couple energy into the attached structure, or sensors which respond to strain coupled from the attached structure.
In different embodiments, particular electrode patterns are selectively formed on the sheet to either pole the PZT plates in-plane or cross-plane, and multiple layers of PZT elements may be arranged or stacked in a single card to result in bending or shear, and even specialized torsional actuation.
In accordance with a further aspect of the invention, circuit elements are formed in, or with, the modular package to filter, shunt, or process the signal produced by the PZT elements, to sense the mechanical environment, or even to locally perform switching or power amplification for driving the actuation elements. The actuator package may be formed with pre-shaped PZT elements, such as half-cylinders, into modular surface-mount shells suitable for attaching about a pipe, rod or shaft.